The present invention relates to a method of producing silicon carbide (SiC). The method is particularly useful for producing silicon carbide heating and lighting elements, high-temperature sensor elements, and finely-divided particles of silicon carbide, (e.g., for use as abrasives, for hardening surfaces, etc.), and the invention is therefore particularly described below with respect to these applications. However, it will be appreciated from the description below that the novel method could also be used for producing silicon carbide for many other applications, such as semi-conductor substrates, hard coatings for turbine blades, high power switching devices, cosmic radiation protectors, etc.
Silicon carbide (SiC), sometimes referred to as carborundum, is a hard, clear, green-tinged or yellow-tinged crystalline compound, which is normally insulating but which becomes conductive when properly heated at a high temperature; for example, when heated to 2000xc2x0 C., it is as conductive as graphite. This material, therefore, is frequently classified as a semiconductor. It is presently used in a wide variety of applications, including abrasives, heating elements, illuminating elements, high-temperature sensors and semiconductor substrates. Because of its highly unique properties, particularly hardness, heat resistance, semiconductivity, thermal and electrical stability, and corrosion resistance, it is commonly considered as the material of the future.
Silicon carbide is generally manufactured, according to one known method, by heating pure silica sand and carbon in the form of coke in an electrical furnace.
According to another known method, a graphite heating element in a cylinder bar is covered with mixture of carbon powder and quartz and high electrical current is passed through it to create a temperature of up to 3000xc2x0 C. At this temperature, the quartz (SiO2) is broken down to pure silicon, which reacts with the carbon powder and creates the required SiC. At a lower temperature zone, a distance from the heater, the SiC begins crystallizing in the shape of small scales. These scales are ground to form a powder of the required size. This process of SiC powder synthesis which takes place in a vacuum (10xe2x88x923 Torr), requires in the order of 36 hours, as well as high electrical currents. Moreover, it is difficult to obtain a powder of the required grain size with this process.
Approximately 45 years ago a new concept was proposed by Lely for growing silicon carbide crystals of high quality; and approximately 20 years ago, a seeded sublimation growth technique was developed (sometimes referred to as the xe2x80x9cmodified Lely Techniquexe2x80x9d. The latter technique lead to the possibility for true bulk crystal preparation.
However, these techniques are also relatively expensive and time-consuming, such that they impose serious limitations on the industrial potential of this remarkable material. In addition, silicon carbide heating or lighting elements prepared in accordance with these known techniques generally vary in resistance with temperature, and/or lose power with age, thereby requiring extra controls, special compensations, and/or frequent replacement.
An object of the present invention is to provide a new method of producing silicon carbide having advantages in one or more of the above respects.
According to a broad aspect of the present invention, there is provided a method of producing silicon carbide (SiC), comprising: introducing into the interior of a furnace a graphite crucible containing a quantity of elemental silicon, and a quantity of elemental carbon free of water, at least the elemental silicon being in the form of finely-divided particles, the quantity of silicon being in excess of the quantity of carbon by weight; subjecting the interior of the furnace to a vacuum; and heating the silicon and carbon to a temperature of 1500xc2x0 C.-2200xc2x0 C. to vaporize the silicon and to convert the carbon to silicon carbide, the crucible being at least partly open at its upper end to the interior of the furnace to permit excess silicon vapors to escape and thereby avoid deposition of silicon on the outer surface of the silicon carbide.
Preferably, the carbon is either lignite carbon or anthracite carbon ground to a fine talc or power form.
During this heating process, the silicon vaporizes, diffuses into the carbon, and converts it to silicon carbide (SiC). Since silicon carbide has a green-tinged or yellow-tinged color, depending on impurities or dopants therein, the formation of such a color during the above-described heating process indicates that the resulting product is indeed silicon carbide.
Since the novel method utilizes elemental silicon, rather than SiO2 (as in sand, glass or quartz), it does not require the high temperatures (e.g., the order of 3000xc2x0 C.), or the long heating time (e.g., the order of 36 hours) required on the prior art process as described above.
As will be described more particularly below, the method may be used in a wide variety of applications for producing shaped articles of silicon carbide, or for producing finely-divided particles of silicon carbide. Thus, the shaped articles of silicon carbide produced in accordance with the invention could be used for manufacturing heating elements, illuminating devices, high-temperature sensors, semiconductor substrates, and the like. The silicon carbide particles produced in accordance with the invention could be used as abrasives, for hardening surfaces such as turbine blades, and the like.
According to another aspect of the present invention, there is provided a method of producing a shaped article of silicon carbide (SiC), comprising: preparing a mixture of a quantity of carbon in the form of finely-divided particles mixed in a binder; shaping said mixture according to the desired shape of the article; applying finely-divided particles of silicon over the outer surface of said shaped article; introducing the shaped article with the finely-divided particles of silicon thereover into a graphite crucible, and introducing the crucible into the interior of a furnace; subjecting the interior of the furnace to a vacuum; and heating the interior of the furnace to a sufficiently high temperature and for a sufficiently long period of time until a resulting product is produced having a green-tinged or yellow-tinged color, the crucible being at least partly open at its upper end to the interior of the furnace to permit excess silicon vapors to escape and thereby avoid deposition of silicon on the outer surface of the silicon carbide.
In the preferred embodiments of the invention described below, the quantity of silicon is in excess of the quantity of carbon by weight to assure relatively complete conversion of the carbon to silicon carbide, with the excess silicon being removed by removing the silicon vapors during the diffusion process to prevent or minimize condensation of the silicon vapor on the outer surface of the silicon carbide.
Where heating or lighting elements are to be produced, the initial composition preferably includes relatively pure silicon but having traces of a dopant, such as zinc, aluminum, tellurium, or another element in the third or fifth column of the periodic table, in the ratio of about 1:10xe2x88x926. The vacuum is preferably from 10xe2x88x921 to 10xe2x88x921 Torr, and the heating temperature is preferably 1550-1600xc2x0 C. Such a process produces silicon carbon heating (or lighting) elements which are green-tinged in color, and have a relatively low internal resistance in the order of tens to a few hundreds of Ohm-cm.
On the other hand, where high-temperature sensors are to be produced, the initial composition preferably includes at least 10% more silicon than carbon, with the silicon being relatively free of dopants; and the heating is preferably effected at a vacuum of higher than 10xe2x88x921 Torr and at a temperature of about 1700-1800xc2x0 C. in order to assure a Si:C ratio of 50:50 and to remove the extra silicon vapors. This technique produces high-temperature sensors having relatively high internal resistance, in the order of hundreds of Kilohm-cm, and a yellow-tinged color.
In some described preferred embodiments, the mixture is prepared by mixing the finely-divided particles of carbon in a water solution of sucrose, and in other described preferred embodiments, the mixture is prepared by mixing the finely-divided particles of carbon in polyvinyl acetate. In both cases, the carbon mixture is prebaked at about 500xc2x0 C. in order to remove all water and to harden the sample. It will be appreciated, however, that other binders may be used.
According to yet another embodiment of the invention described below, the method produces finely-divided particles of silicon carbide (SiC), by: introducing into the furnace the silicon and carbon in the form of layers of finely-divided particles separated from each other by a layer permeable to silicon vapor.
Further features and advantages of the invention will be apparent from the description below.